This invention relates to a supply assembly for a LED lighting module, comprising a supply unit to which the LED lighting module can be connected and a burst control unit which is capable of switching the power supply (the voltage or the current) to the LED lighting module on and off with a burst frequency so as to control the mean light output of the LED lighting module.
LED lighting modules are being used more and more to illuminate objects. They are also used as signal lighting, for example, in traffic lights. The light output of a LED lighting module can be controlled in various well-known ways. One of said ways consists in controlling (dimming) the current intensity through the LED module. This has the drawback that at a current intensity below approximately 20% of the nominal current intensity, the relation between the current intensity and the light output becomes very non-linear, and the efficiency of the module becomes far from optimal. Therefore, solutions have been sought which enable the light intensity to be controlled while the nominal current intensity through the module is maintained. U.S. Pat. No. 5,661,645 describes a supply unit for a LED lighting module as described in the opening paragraph. In said document, the burst control unit is arranged between an output of a voltage source and the LED lighting module, so that the power supply to the LED lighting module can be switched on and off with a certain burst frequency. The voltage source is continuously in the on-state even when the power supply to the LED lighting module is cut off, which results in an unnecessary loss of energy.
It is an object of the invention to provide an efficient, reliable, inexpensive and simple supply assembly for a LED lighting module with an improved efficacy.
This is achieved in accordance with the invention in that the burst control unit is capable of switching the power supply unit on and off with the burst frequency. In this case, the supply unit is in the on-state only when current is actually being supplied, so that the generation of heat and energy losses are minimized, resulting in an improved efficacy of the system.
Preferably, the burst control unit comprises a pulse-duration modulator which is capable of controlling the length of the pulses during which the supply unit is in the on-state. This enables the mean light output to be controlled.
Preferably, the supply unit is a controlled constant-current source. This has the advantage that further measures in the LED module, such as the provision of additional resistors, are not necessary to control the electric current through the LEDs.
Preferably, the supply unit is a high-frequency switch mode power supply (smps) which supplies a ripple DC current or a ripple DC voltage, said supply unit preferably comprising a half-bridge converter, a buck converter or a flyback converter. The pulse duration modulator can preferably switch the converter on and off with the burst frequency. In this way, the desired effect can be achieved very efficiently. In addition, preferably the ripple frequency of the converter is an integer multiple of the burst frequency, and the ripple DC voltage or ripple DC current can be synchronized with the burst control signal, resulting in a reduction of any stability problems and loss of energy in the system.
For the same reason, the burst frequency preferably is an integer multiple of the frequency (customarily 50 Hz or 60 Hz) of the voltage source, for example the mains voltage to which the supply assembly can be connected, and the burst control signal is synchronized with the mains voltage signal.
Preferably, the burst frequency lies in the range between 50 and 200 Hz or above 20 kHz; more preferably the burst frequency is 100 Hz (if the mains frequency is 50 Hz) or 120 Hz (if the mains frequency is 60 Hz). When use is made of a half-bridge converter, it is best if the burst frequency is chosen as low as possible in connection with switching losses. However, the frequency should preferably exceed the frequency that can be observed by the human eye (approximately 70 Hz), because at a lower frequency the flicker produced by the LEDs is noticeable, which is annoying. In addition, the frequency should preferably be outside the human range of hearing of 200 Hz to 20 kHz to preclude disturbing sound effects.
Preferably, the current supplied by the supply unit in the on-state to the LED lighting module is a constant current which has been chosen as a nominal current, and which results in an optimum light output of the LED lighting module, so that the supply unit can be optimized for the nominal current intensity, which has a favorable effect on the efficacy.
In a preferred embodiment, the burst control unit is coupled to a sensor which is capable of measuring the temperature and/or the light output of the LED lighting module, said burst control unit being capable of controlling the length of the pulses during which the supply unit is in the on-state in dependence upon the value measured by the sensor. By virtue thereof, the light output of the module can be stabilized under varying conditions. In a further preferred embodiment, the duration of the pulses can be adjusted by a user. By virtue thereof, the light output of the module can be dimmed in accordance with the user""s needs.
The supply assembly as herein disclosed is also suitable to operate a LED lighting module which forms part of a display unit, for instance a liquid crystal display (LCD).